I just puchased a Kubota L3700SU and didn't get the backhoe.
I have a neighbor that will let me use his International 3 point hitch back hoe but I dont know where to hook up the inlet & outlet hoses too. Is there some kind of hydraulic box attachment to buy? What would it be called and how much am i looking at spending.

The international backhoe's that I've seen were normally quite heavy unlike those normally used on a compact tractor.
I'd say you could be getting yourself into trouble as far as the tractor goes with this unit on it.

All Kubota hoe's at this point have subframes and I think that has been there strong point for not having chassis issues as some of the other companies although they seem to also be having issues with just loaders as well.

Back to your question, not knowing how that hoe is hooked up to your neighbors tractor makes it harder to tell you what you need if you do decide to do it.